Lamps and optical systems for a variety of lighting applications, such as medical, theatrical, educational, merchandising lighting, include a cold light mirror. A cold light mirror transmits the infrared portion of the light striking the cold light mirror surface and reflects all or selected portions of the visible wavelength bands striking the reflector surface. By transmitting the infrared (i.e., heat) portion of the light through the cold light mirror surface, the light beam projected from a light source/cold light mirror combination is cooler than a light beam projected from a light source/metal mirror combination.
A cold light mirror which reflects substantially all of the visible light is often referred to as a neutral cold light mirror. A cold light mirror which reflects only a portion of the visible light is often referred to as a color-correcting cold light mirror. However, both are cold light mirrors.
Neutral and color-correcting cold light mirrors typically used in lamps and optical systems are described extensively in the technical literature. Such cold light mirrors usually include a transparent form or substrate, one surface of which is coated with a dichroic coating consisting of a series of thin film layers of alternating high and low refractive index materials (compared to the substrate). The optical thickness of each layer is generally one-quarter (1/4) of the wavelength of the radiation for which high reflectance is desired.
In lighting applications utilizing high wattage lamps, heat transmission through a cold light mirror into the lamp fixture or housing can be detrimental to the lamp fixture or housing. Such heat transmission is especially a problem for closed fixtures in which the temperature may rise above the maximum allowable service temperature for the fixture or housing. Such raised temperature may result in degeneration of the wire insulation and/or arts of the fixture fabricated from heat sensitive materials, such as plastic.
U.S. Pat. No. 3,944,320 of McLintic is concerned with thermal expansion and cooling problems associated with use of cold light mirrors on transparent substrates in light beam projection applications. McLintic describes a cold light mirror having a metal substrate, a pigmented vitreous coating on the substrate which coating is capable of absorbing infra-red radiation and having a smooth glazed surface, and a dielectric interference coating on the surface of the vitreous coating. McLintic emphasizes that, in manufacturing the cold light mirror of his invention, care has to be taken to ensure that the glaze of the pigmented vitreous coating is specular in order that the optical properties of the complete structure are unimpaired. Such special care is required because the layers of the interference coating are too thin to be able to remove surface irregularities which may exist on the surface upon which they are laid. Further, the absorption of heat by the reflector does not overcome problems caused to the fixture or housing by heating of the fixture or housing.
It would represent an advance in the art to obtain a light mirror which reduces the heat absorbed by or transmitted through the coated reflector.